In the publication "Kieffer and Benesovsky", HARDMETALS, Springer-Verlag, Vienna-New York 1965, pages 202 to 216, WC--TaC (NbC)--Co hard metals and WC--TiC--TaC (NbC)--Co hard metals are indicated to be especially suitable for the machining of steel workpieces. These alloys are composed in the broadest ranges of 35 to 80% WC, 5 to 45% TaC, 0.5 to 30% TiC and 1 to 30% binder (iron, cobalt, nickel) and have a greater ductility than pure WC--TiC--Co alloys and greater cutting resistance. Such hard metals are useful as hard metal basis bodies which are to be coated with titanium carbide, titanium nitride and/or aluminum oxide (compare DE-B-22 63 210, DE-C-22 537 45). The coatings are applied according to the state of the art, for example, by the so-called CVD (Chemical Vapor Deposition) process to increase the wear resistance of the cutting body.
Nevertheless, the known coatings have a detrimental effect upon the ductility, i.e. the brittleness of the cutting body is increased. The basis for this (increase in brittleness) are tensile stresses which develop after cooling of the hard material layer applied at about 1000.degree. C. by CVD and leading to cracks in the coating.
With higher mechanical stresses, therefore, there is the danger that cracks in the surface layer will propagate in the hard metal base body and finally give rise to breakout at the cutting edge. To avoid or to limit this disadvantageous effect, it has already been proposed to provide the hard metal base body with a ductility-increasing boundary zone. This zone close to the surface has a low content of cubic mixed carbides and carbonitrides or is free from cubic mixed carbides and carbonitrides and is enriched with such the binder metal as cobalt. As a consequence, the boundary zone has a greater ductility than the mixed carbide containing hard metal base body. The propagation of cracks from the coating into the hard metal is therefore retarded.
From DE 32 11 047 A1 and U.S. Pat. No. 4,548,786, basically two processes for producing the boundary zones enriched with cobalt and low in mixed carbides are known.
In the first process, nitrogen containing such compounds as, for example, titanium nitride or carbonitride are added to the mixture forming the hard metal in given quantities (compare DE 32 11 047 A1). With vacuum sintering of such mixtures at high temperatures between 1300.degree. and 1500.degree. C. diffusion processes take place which effect the described boundary zone modification.
In a further proposal (compare U.S. Pat. No. 4,548,786), one starts with a conventional mixture of hexagonal tungsten carbide, cubic carbides and cobalt and subjects and the mixture to the compacting sintering below the melting temperature of the binder metal at about 1250.degree. C. to a treatment in nitrogen gas. The nitriding is so carried out that sufficient nitrogen is taken up in a short time by the cubic carbides to partially convert the cubic carbides into carbonitrides. In the subsequent sintering at a higher temperature in vacuum, the desired boundary zone effect is achieved as in the previously described process, whereby the boundary zone thickness lies between 10 to 50 .mu.m.
The aforedescribed processes have, however, the disadvantage that many parameters influence the overall hard metal fabrication from the completion of the mixture through the pressing process to the final sintering and thus influence the desired result.
As a consequence, many cutting bodies must be subjected to milling following the sintering and then to aftergrinding with the requisite precision. As the grinding operation again removes the relatively thin boundary zone, the aforedescribed process cannot be used for ductility increase where the cutting bodies are highly stressed mechanically by the milling.